It is common practice to provide a motor grader with one or more hydraulic motors which are connected for driving an internal annular gear forming part of a circle structure to which the grader blade is attached and rotated to change its angularity relative to the direction of travel of the motor grader. Heretofore, the hydraulic motors provided for performing this rotate function have been fixed displacement motors. However, it is desirable to be able to rotate the blade at a high speed when placing it in a mirror image position when the grader is being turned to reverse its direction of operation. Also desirable is the ability to apply a high torque for turning the blade when the blade is in ground contact. High speed and high torque functionality are not usually required at the same time.
The provision of a fixed displacement motor which can both operate to drive the circle at a satisfactory high speed and to impart a satisfactory torque to the blade creates some tradeoffs in the circle drive design. For example, the need for high torque and high speed makes the total hydraulic horsepower required for this function much higher than the other functions on the motor grader. Tradeoffs for keeping the total hydraulic system in balance may include reducing hydraulic flow for effecting rotation of the circle, thus requiring a smaller control valve. The overall pump size could also be reduced if the flow for effecting rotation of the circle was reduced, or the pump size could remain the same with an overall improvement in flow availability.
It is desired then to be able to drive the circle of a motor grader at a high speed or at a high torque without necessitating an increase in the size of the pump or control valves used for the circle drive function.